Tony Blair is due to join the Irish Taoiseach, Bertie Ahern, and pro-Good Friday agreement parties for a fourth day of talks at Weston Park, which is on the Shropshire/Staffordshire border, in a bid to stabilise the Northern Ireland political institutions.

Sinn Fein's president, Gerry Adams, this morning rounded on the acting Northern Ireland deputy first minister, Seamus Mallon, for claiming republicans had given the other parties no indication of their position on decommissioning.

"He doesn't seem to realise that the elections are over. He needs to get real and stop being grumpy and he needs to fulfil his responsibility even in the acting capacity of deputy first minister.

"Seeking to score points off us in public while refusing to engage with us in private is not the way to make this process."

Mr Mallon had reported before leaving on Wednesday that progress was being made on the policing and demilitarisation issues but that republicans were giving nothing away on disarmament.

Mr Adams' criticism of his nationalist rival was seen as further evidence of the sour atmosphere among the Northern Ireland parties.

Despite the deadlock over the issue of IRA arms, Downing Street said they hoped to conclude the talks today, but stressed "we don't talk deadlines".

"We have already had three days of hard discussions. The prime minister wouldn't be going back unless he believed there was a point, and there was a prospect of making progress," Downing Street said.

Last night, nationalist rioters attacked RUC officers as they tried to clear a path for some 100 Orangemen returning home past the Catholic Ardoyne area from yesterday's annual Belfast July Twelfth parade.

Two blast bombs and 263 petrol bombs were thrown at police during what they called a sustained attack from over 200 nationalist youths who tried to force their way onto the Crumlin Road to block the Orangemen's route.

Police in full riot gear deployed water cannon and fired 48 rounds of plastic bullets during the disturbances.

Two officers were set alight by petrol bombs and one was attacked with a pickaxe, although he was later discharged from hospital.

Nineteen officers required hospital treatment and two officers remain in hospital for treatment, the RUC said.

The RUC assistant chief constable, Alan McQuillan, condemned the violence and said the nationalists' sustained attack on RUC officers was "quite clearly" orchestrated and pre-planned.

"We were attacked with paint bombs, acid bombs and petrol bombs. That was quite clearly orchestrated and we have had some of the heaviest rioting we have seen in that area for some time," he said.

The RUC's chief constable, Sir Ronnie Flanagan, said that last night's violence was dreadful and "clearly not spontaneous" and defended officers' use of water cannon as "absolutely necessary".

On the nationalist side, however, there were claims that a number of people had been injured by police. Gerry Kelly, a Sinn Fein assembly member, claimed protesters were baton charged off the road and accused the security forces of turning on stewards who were trying to police the protest.

"This is a Catholic area, this is not here to protect Catholics, this is not here to protect nationalists, this is to ram a parade right through this area," Mr Kelly said.

Two loyalist paramilitary-linked parties, the Progressive Unionists and Ulster Democratic party, will not join in the peace discussions, which broke up on Wednesday with signs of some progress on securing nationalist support for police reforms and demilitarisation.